The hardness and toughness of tungsten carbide compositions have made such materials well suited for use in cutting tools and metal forming tools as well as in surface coatings for devices requiring hard, durable surfaces. Typically, a mixture of tungsten carbide and a binder, such as cobalt, chromium, titanium, nickel, and combinations thereof, is pressed into a mold under high pressure, followed by sintering to fuse the mixture and thereby form an article. Tungsten carbide surface coatings are generally formed by processes of coating substrates with tungsten carbide compositions using techniques such as spraying, chemical vapor deposition, physical vapor deposition, sputtering, or plasma deposition.
The articles and surfaces thus formed generally require polishing or grinding to provide smooth surfaces and to obtain precise dimensions for the article or surface. The hardness and toughness of tungsten carbide compositions that makes tungsten carbide so useful provides special challenges in polishing processes. Typically, diamond grit has been used for the polishing of tungsten carbide. Micron-sized particles of diamond grit are either imbedded in a polishing or grinding wheel or imbedded in abrasive films such as are available from the 3M Company. Various processes have been developed to utilize diamond grit in the polishing of tungsten carbide surfaces. U.S. Pat. No. 6,004,189 describes a process of polishing a tungsten carbide surface involving a complex series of operations including oscillating a diamond abrasive film against the surface of a rotating cylinder coated with tungsten carbide.
In machining processes, typically a cutting edge, such as a cutting tool insert, is brought into contact with a metal workpiece to be machined and moved relative to the workpiece to remove metal from the surface of the workpiece. Cutting tool inserts are used in both turning processes, wherein metal is removed from a cylindrical workpiece, and milling processes, wherein metal is removed from a noncylindrical workpiece. As the cutting tool insert cuts into the surface of the workpiece, the insert separates a thin layer of the workpiece surface which moves against the flank face of the cutting tool insert forming a chip. The bottom surface, or the rake face, of the cutting tool simultaneously moves across the newly-formed surface of the workpiece as the surface is machined. Thus, cutting tool inserts having improved surface smoothness are expected to provide for reduced friction with the surface being machined, thereby reducing frictional heating of the workpiece during machining, and further resulting in improved surface quality.
Although tools and surfaces made of tungsten carbide compositions possess exceptional hardness and toughness, surface wear is inevitable and eventually requires reconditioning or replacement of the tool or surface. Often surface coatings such as titanium nitride are applied over tungsten carbide to reduce friction and protect the surface, but such coatings also wear, exposing the underlying tungsten carbide to surface damage. Reconditioning then requires an extra step of stripping the coating followed by regrinding of the surface, increasing the cost of reconditioning.
Despite the utility of diamond-based abrasive methods for the polishing of tungsten carbide articles and surfaces, the expense of diamond coupled with limits for the smoothness achievable by diamond-based abrasives compromises the quality of machining processes and the economics of tool reconditioning processes. Thus, there remains a need for improved methods for the polishing of surfaces comprising tungsten carbide. There also exists a need for cutting tools having improved surface quality for use in the precision machining of workpieces.
The invention provides such a method for the polishing of surfaces comprising tungsten carbide and provides improved cutting tool inserts. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.